1. Field of the Invention
The present invention relates generally to an angle sensor and, more particularly, to an angle sensor, which is capable of accurately measuring an angle of a steering shaft by selecting an optimum gear ratio between main and subsidiary gears constituting the angle sensor.
2. Description of the Related Art
Generally, an auxiliary steering system supported by additional power is used as a device for ensuring the steering stability of a vehicle. Conventionally, the auxiliary steering system uses a hydraulic pressure. However, recently, an electronic power steering system, which consumes less power and is excellent in accuracy, has been used.
Such an electronic power steering system (EPS) drives a motor by an electronic control unit depending on driving conditions detected by a vehicle speed sensor, an angle sensor and a torque sensor, thus ensuring stability in turning and providing a rapid restoring force, and thereby allowing a driver to safely drive a vehicle.
The angle sensor is a device that outputs an electric signal in proportional to a rotation angle of a steering shaft. Recently, the angle sensor is frequently combined with the torque sensor measuring steering torque to form an assembly.
FIG. 1 is a plan view illustrating an example of the angle sensor.
In the case of the angle sensor, as a driver rotates a steering wheel, a main gear 20 attached to a steering shaft rotates in conjunction with the steering wheel, and there occurs a difference in rotation angle between input and output shafts of the steering shaft. At this time, a magnetic device detects magnetic fields and rotating directions of magnets 31 and 32 attached to subsidiary gears 21 and 22 engaging with the main gear 20, and thereafter sends a detected signal to an electronic control unit.
The main gear 20 and the subsidiary gears 21 and 22 are configured to have a predetermined gear ratio. Commonly, the number of teeth formed on an outer circumference of the main gear 20 is greater than that of each subsidiary gear 21 or 22.
For example, as in the general case, if the gear ratio of the main gear to the subsidiary gear is 2:1, the subsidiary gear rotates twice when the main gear rotates once. Digital output of a Hall sensor detecting a change in magnetic field of the magnet is multiplied by a gain corresponding to a predetermined scale factor, and thus a rotation amount is measured using a 14-bit timer counter.
Here, since the number of rotations of the main gear must be detected via the number of rotations of the subsidiary gear, the scale factor may affect accuracy in measuring the number of rotations of the main gear. Consequently, the conventional angle sensor is problematic in that a value of the scale factor is large, so that an error in measurement of a rotation amount occurs.